1. Field of the Invention
This invention relates generally to a turntable assembly for phonograph records, and more particularly is directed to improvements in the means by which the phonograph records are supported on the rotatable turntable of a phonograph record player.
2. Description of the Prior Art
Phonograph pickup cartridges are being continuously improved by being made lighter in weight and capable of playing or reproducing phonograph records with decreased vertical contact pressures of the stylus in the record groove. For example, cartridges are presently available that can play phonograph records quite well with a vertical contact force of only one-half gram, and it is anticipated that, in the near future, cartridges will be available to play phonograph records with a vertical contact force of only about one-tenth gram. When using such cartridges, it is important that some means be provided to permit the playing of phonograph records that are not perfectly flat, that is, records that are warped or distorted from a flat plane. Since phonograph records are made of plastic materials which deform under continuous stress and exposure to elevated temperatures, such phonograph records may become distorted or warped either by reason of improper manufacture or because they are not stored under ideal conditions. When a warped or distorted phonograph record is placed on an existing turntable, the undersurface of the record does not contact the surface of the turntable over its entire area so that gaps or spaces are left between the record and turntable with the result that the record is at least partly in a floating or unstable condition. During reproduction of the recorded signals of such a distorted or warped record, the tracking of the needle or stylus of the cartridge in the record groove and generates considerable forces which react on the phonograph record. Due to the unstable floating conditions of the warped record, such forces are apt to produce vertical vibration of the record at a resonance frequency of the latter. Accordingly, the frequency characteristics of the reproduced signal are disturbed, particularly in the vicinity of such resonance frequency, and such disturbance of the frequency characteristics cannot be corrected or removed even though the remainder of the sound reproducing system, such as, the turntable, pickup arm, amplifier and speakers may be components of high quality. The foregoing problems are particularly disturbing in the case of the reproduction of stereo records in which vertical vibrations of the stylus are intended to correspond to right-left difference signals. By reason of the vertical vibrations of a warped or distorted record at its resonance frequency, sounds which originated at the center may be displaced toward the right and left in the reproduced stereophonic sounds so that the stereophonic effect is distorted.
Although conventional phonograph record turntables are usually provided with a rubber mat having concentric circular grooves or ridges on its upper surface, such rubber mats are obviously not adapted to ensure uniform supporting contact with the undersurface of a record over the entire area of the latter when the record is substantially warped or distorted. Further, although the existing rubber mats on phonographic record turntables are intended to absorb vibrations transmitted to the turntable, for example, by the drive assembly for the latter, such rubber mats do not fully absorb or damp the vibrations which are, at least to some extent, transmitted to the record and then picked up by the cartridge so as to be reproduced as corresponding noise signals.